[webvtt] Pull Request: Add WebVTT header metadata to disambiguate metadata cues (#511)

cookiecrook has just submitted a new pull request for https://github.com/w3c/webvtt:

== Add WebVTT header metadata to disambiguate metadata cues (#511) ==
Draft: Claude-assisted PR. Not yet ready for review.

Replaces the ATTRIBUTES-block approach explored in #523 with the TTWG-consensus design: file-level key/value pairs are written on the lines immediately following the WEBVTT file header line and are terminated by a blank line, rather than living in a separate block.

Key behaviors per the April 23 + May 7 TTWG meeting consensus and follow-up review comments:

- Reserves four header keys: lang, kind, label, type (all optional; lang aligns with HTML <track srclang>; type addresses #511 by identifying metadata schemas, with the taxonomy work continuing in #512).
- Accepts both ":" and "=" as separators; "=" is permitted for HLS-compatible files but is marked non-recommended.
- Matches reserved keys case-sensitively; authors must use lowercase.
- Allows Unicode in keys and values, excluding bidi controls, line breaks, and the "-->" substring.
- Requires non-reserved keys to contain a hyphen and not start with one, reserving the hyphen-free namespace for future standardization.
- Parses leniently: invalid lines are ignored individually and do not invalidate the rest of the header; no parser warnings are required.
- Adds the parser plumbing (a new |header metadata| slot on the parser signature, a "collect WebVTT header metadata" algorithm, and a new return path from "collect a WebVTT block" when the in-header flag is set).
- Updates the WebVTT metadata text prose to recommend declaring kind and type for files delivered outside an HTML context, addressing the cue-format ambiguity from #511.

Closes #511. Defers HTML-integration details (precedence between VTT header metadata and <track> attributes, processing model) to whatwg/html#11665.

See https://github.com/w3c/webvtt/pull/548


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Received on Thursday, 28 May 2026 21:11:26 UTC